1. Field of the Invention
The present invention relates to a method of changing a telecommunication channel in a mobile communication system for a given mobile station, which mobile station has a connection in progress, and the connection is provided with at least a channel on a radio connection between the mobile station and a base station system and a telecommunication channel between the base station system and a mobile services switching centre, and in which method: a handover message is received. The invention further relates to a mobile communication system comprising: a mobile services switching centre, a base station system, a speech processing unit communicating with the base station system through first telecommunication channels and with the mobile services switching centre through second telecommunication channels, in which case a predetermined correspondence is defined for the first and second telecommunication channels, and a mobile station having a connection in progress through the base station system and the mobile services switching centre, in which case the ongoing connection of the mobile station is provided at least with: a channel of a radio connection between the mobile station and the base station system, and at least one of the first telecommunication channels and at least one of the second telecommunication channels for transmitting telecommunication signals associated with the ongoing connection of the mobile station between the base station system and the mobile services switching centre.
2. Description of Prior Art
The present invention relates to the changing of a telecommunication channel in a data transmission network between a base station system and a mobile services switching centre, particularly in a digital mobile communication system. In this application, the concept base station system means the entity which is formed of base stations and their controller means, such as a base station controller.
The frequency spectrum required by a digital mobile communication system, such as the GSM system (Global System for Mobile Communications), depends on the transmission rate used on a radio path. The higher the transmission rate used on the radio path, the wider the frequency spectrum required. Consequently, speech encoding of a fixed PSTN/ISDN telephone network (Public Switched Telephone Network/Integrated Services Digital Network), used for transmitting digitally encoded information 64 kbit/s for each traffic channel, is not normally suitable for use on the radio path. For this reason, instead of speech encoding of the fixed network, RPE-LTP speech encoding (Regular Pulse Excitationxe2x80x94Long Term Prediction) is used on the radio path of the GSM system, for example.
In order to adapt different encoding schemes and different data transmission rates, a speech processing unit (a transcoder) is required between the base station system and the mobile services switching centre. In other words, in the GSM system, for example, the interfaces for the transcoder are 64 kbit/s towards the mobile services switching centre and 8 or 16 bits/s towards the base station system. In the GSM system, the first telecommunication channels, i.e. the connections between the transcoder and the base station system, are called Ater lines. Correspondingly, the second telecommunication channels, i.e. the connections between the transcoder and the mobile services switching centre, are called A interface lines. A predetermined correspondence has been defined for the first and second telecommunication channels, i.e. it has been defined beforehand that the given A interface line corresponds to the given Ater line (or given Ater lines).
In a known mobile communication system of the above type, a need occasionally arises to change for an ongoing connection, such as a call, the telecommunication channels used between the base station system and the mobile services switching centre. Such a need may arise when a call type has changed, when an encoding scheme for a call is wished to be changed (in which case the call is to be directed to another speech processing unit) or when the use of the speech processing unit employed is wished to be optimized, for example.
In known mobile communication systems, the above-mentioned telecommunication channels are changed by handover which is carried out to the connection in question. In that case, for instance the signalling channel and the first and second telecommunication channels between the base station system and the mobile services switching centre change. In addition, the channel of the radio connection between the mobile station and the base station system change. Consequently, such handover requires much signalling by the mobile station and other elements in the system before it can be completely accomplished. However, somewhat excessive resources are required for this kind of known handover, since no advantage is necessarily gained by the change of the channel of the radio connection.
An object of the present invention is to solve the above problem and provide a solution by means of which the telecommunication channel between the base station system and the mobile services switching centre can be changed in an easier and more resource-saving manner. This objective is achieved by the method of the invention which is characterized in that the method comprises the steps of: putting a new telecommunication channel between the mobile services switching centre and the base station system, indicated by the handover message, into use for the connection, and connecting the channel of the radio connection used by the connection to the new telecommunication channel such that the channel of the radio connection remains unchanged irrespective of the change of the telecommunication channel between the base station system and the mobile services switching centre.
The invention also relates to a mobile communication system to which the method of the invention can be applied. The mobile communication system of the invention is characterized in that the mobile services switching centre comprises means for receiving a handover message and means for putting a new second telecommunication channel indicated by the handover message into use for the connection indicated by the handover message, and that the base station system comprises means for transmitting the handover message to the mobile services switching centre, and means for taking a new first telecommunication channel, which corresponds to the new second telecommunication channel indicated by the handover message, into use for the connection indicated by the handover message, in which case the base station system is arranged to connect the new first telecommunication channel to the channel of the radio connection assigned to the mobile station in such a way that the channel of the radio connection remains unchanged irrespective of the change of the first and second telecommunication channels.
The invention is based on the idea that it is possible to change the telecommunication channels between the base station system and the mobile services switching centre in an extremely simple and resource-saving manner when the system is provided with, in addition to a more complete conventional handover operation, a second handover procedure which can be implemented by fewer resources. This is achieved when the base station system is arranged to indicate a new channel for the ongoing connection to the mobile services switching centre directly in the handover message. Such a new handover procedure makes it possible that the channel of the radio connection between the mobile station and the base station system needs not be changed, for example, whereby the mobile station does not need to be in any way aware of the change of the telecommunication channel. Furthermore, the invention makes it possible that the existing logical SCCP signalling channel (Signalling Connection Control Part) between the mobile services switching centre and the base station system can be kept unchanged, i.e. it is not necessary to change it to a new signalling channel as in known handovers. The most significant advantage of the solution of the invention is thus that the telecommunication channel between the base station system and the mobile services switching centre can be changed as quickly as possible in a most resource-saving manner in which the amount of the required signalling has been minimized.
The preferred embodiments of the method and mobile communication system of the invention are disclosed in the accompanying dependent claims 1 and 2 to 6.